Recently, the applicability of neurons to electronic devices has been vigorously studied as well as the medical study. An action potential is generated in a neuron which is in an active state. A change in the ion permeability of a neuron leads to changes in intra- and extracellular ion concentrations which are responsible for generation of an action potential. Therefore, if a potential change in association with a change in ion concentration around a neuron is measured, the activity of the neuron can be monitored.
The above-described action potential associated with the neuron activity is conventionally measured by placing an electrode of glass or metal (e.g., platinum) for measuring an extracellular potential, around a cell with the aid of a micro-manipulator or the like. Alternatively, a similar electrode is inserted into a cell so as to measure the action potential of the cell.
These conventional techniques have the following disadvantages: skill in electrode preparation is required; the electrode has high impedance and therefore the signal is susceptible to external noise; and cells or tissues are injured if an electrode is inserted into the cell. Therefore, conventional electrodes are not suitable for long-term monitoring.
To avoid such problems, the inventors have developed a multiple electrode including a plurality of micro-electrodes made of a conductive material provided on an insulating substrate, and a lead pattern, on which cells or tissue can be cultured (Japanese Laid-Open Publication No. 6-78889, and Japanese Laid-Open Publication No. 6-296595). With this multiple electrode, the activities of neurons can be monitored without injuring cells or tissue for a long period of time. In the above-described multiple electrode, an uppermost surface of the electrode contacting cells is plated with porous platinum black (Japanese Laid-Open Publication No. 6-78889) to adjust the impedance of the electrode to a practical level, i.e., about 50 kΩ or less.
In the multiple electrode, when measuring the action potentials of neurons, for example, a stimulus (a current or a voltage) is externally applied to a neuron located in a site of a tissue, and the response to the stimulus is monitored at another site, thereby making it possible to analyze a neural network in the tissue. In this case, a micro-electrode, which is located in a site most suitable for application of a stimulus, is selected as a stimulus electrode from a plurality of micro-electrodes on the multiple electrode. Any micro-electrode in the vicinity of the stimulus electrode can be used as a reference electrode. A stimulus is applied between the two micro-electrodes, and response potentials of a plurality of micro-electrodes are measured.
However, other cells or tissues are present on the reference electrode in the vicinity of the stimulus electrode and therefore, if a stimulus (a current or a voltage) is externally applied, not only the cell on the stimulus electrode but also other, cells on the reference electrode are stimulated. Therefore, the desired signal is often not accurately measured. Further, if the impedance of the reference electrode is increased, artifacts due to external noise or stimulus cannot be prevented from being increased.